Pollutants from booming farms combined with record wet weather are contaminating the nation's mightiest waterway.

Flowing 2,300 miles from Minnesota to the Gulf of Mexico, the Mississippi River provides drinking water, food and jobs for millions of people. A journey downriver reveals how the agricultural industry is contributing to one of the nation’s biggest ecological disasters.

The farmland that surrounds the Mississippi is more productive than ever, supplying the world with cheap crops and meat. There is a cost. Fertilizer and manure used on farms contain nitrogen and phosphorus. Flushed into waterways, they can taint drinking water and foster algae that chokes out marine life. Intense rains and flooding this spring exacerbated the problem.

Expanding Croplands

Mississippi Headwaters, Minn.

The Mississippi begins as a stream flowing from a lake in a forest wilderness, crystal clear and toxin-free. Contaminants are accumulating in rivers and drinking water below those headwaters, after burgeoning global demand for food led farmers to convert tens of thousands of acres of forest, grassland and dormant fields into cropland.

Farmers near the Straight River in recent years have installed sprinklers on once fallow land to grow fertilizer-hungry row crops in sandy soils. Irrigated acres near the river increased 81% in the 24 years ending in 2016.

Concentrations of nitrate are rising in the Straight. During the summer, they measure at least 100 times higher than in nearby rivers.

Farm Runoff

Southern Minnesota

Farther south, the Minnesota River is overwhelmed by farm runoff. Corn and soybean fields fill southern Minnesota. The region has a growing hog industry. Lakes and rivers in southern Minnesota are often too polluted to fish or swim in, and the river has become the state’s biggest contributor of nitrates to the Mississippi.

Drainage Problem

Iowa

In Iowa, the most productive farm state along the river, even more land is intensively farmed — 70% of the state’s surface area. Much of that cropland is managed using tile drainage, networks of underground piping that carry water away from fields.

This helps farmers manage moisture, but also moves excess nutrients from soil into waterways more swiftly.

Farmers, researchers and officials have increased spending to try to mitigate the impact of runoff on Iowa’s waterways.

But nitrate levels in the Mississippi are trending upward. They oscillate with the volume of water in the river, dropping during dry spells such as a 2012 drought. This year, after spring flooding, the river looks likely to carry more water than average.

Drinking Troubles

Hiawatha, Kan.

As farming has intensified, nitrates have also tainted wells in a Kansas town near the Missouri River, a key source of pollutants in the Mississippi. Nitrates have been associated with birth defects, thyroid problems and cancer. Federal officials say water can be unsafe to drink at nitrate levels over 10 parts per million. Hiawatha officials struggled for years to clean up the water, urging pregnant women and infants not to drink it when nitrates spiked. In 2017, Hiawatha began building a $4.5 million treatment plant to remove nitrates from water.

To pay for it, officials raised water rates for Hiawatha’s 3,200 residents.

Chicken Craze

Arkansas

Before reaching the Gulf, the river flows between Arkansas and Mississippi, states that have helped feed the world’s booming appetite for chicken. Arkansas is the second-biggest poultry producer after Georgia.

That has put more nitrates and other contaminants from chicken farms and slaughterhouses into waterways such as the Arkansas River, researchers say, which flows to the Mississippi.

Dead Zone

The Gulf of Mexico

Every summer, nutrients from the Mississippi pour into the Gulf, fueling algae blooms that starve the water of oxygen and kill sea life. Heavy rainfall throughout the Mississippi River watershed this spring led to record-high river flows, boosting nitrate and phosphorus loads. As a result, scientists predict this year’s “dead zone” will total 7,829 square miles, an area roughly the size of Massachusetts, and close to the record set in 2017.

Note:In 2016, the mission to determine low oxygen levels in the Gulf of Mexico was cancelled, so data are not available for that year